Mark
Yes the filter is inside the brass cylinder, the top does unscrew...I always coat the threads in Vaseline after cleaning. I have had to resort to carefully putting the lid in a vice then turning the body to release it.
You need to blast out the dirt in the filter...this needs doing regularly....it only take a few minutes at the end of the day.
Also check the jets...they just twist and pull out...again I blast water through both ways very regularly.
Have fun